Hydraulics

What are Hydraulics?

Hydraulic modeling contains data on the modeled water surface elevations of flooding throughout a floodplain - this same information contained in HEC-FDA Version 1 Water Surface Profiles.  

What is the Purpose of Hydraulics?

Hydraulic modeling is used in HEC-FDA to identify the water surface elevation at each structure for at least 8 unique events. This information is used in calculating the stage-damage functions for each structure, after which the structure-level stage-damage functions are aggregated to stages at the index point which are defined by the combination of the input summary relationships. The hydraulics data can also be used to develop a graphical stage-exceedance probability functions.

Where Do I Get Hydraulics?

Hydraulic engineers will use HEC-RAS or some other hydraulic modeling software to develop the spatial data sets.

Working with Hydraulics in HEC-FDA

Hydraulics Overview

There are two options for importing hydraulic modeling into HEC-FDA:

1. Grids (.tif), or
2. Hierarchical data format (.hdf).

Hydraulic modeling can be exported from HEC-RAS in either format; hierarchical data format is the more convenient option. 

The hierarchical data format export is described in HEC-RAS 2D User's Manual here. If the hydraulic modeling is steady state, one HDF will be imported. If the hydraulic modeling is unsteady state, all HDFs in the model-specific subdirectory will be imported. For example, if the existing condition model was run for 8 events, there should be those 8 HDFs in a dedicated folder and none else. Depth grids will not be handled by the software directly and are not advised for use with the stage-damage compute. 

Gridded data is described in the HEC-RAS Mapper User's Manual here. Whether steady state or unsteady state, the gridded data must be organized in 8 subdirectories, one subdirectory with one .tif for each flood event, similar to how unsteady state modeling is organized.

Differences Between HEC-FDA Versions 1 and 2

Hydraulics data is used the same way in HEC-FDA Version 2 as it was in Version 1 - to calculate stage-damage functions. The main difference in the data used by the two versions is the format. The data consumed by Version 1 consisted of a table of data organized by river mile - a 1-dimensional view of the world. In Version 2, the data is represented within a geospatial data set. In Version 2, the hydraulics modeling does not contain flow information like in Version 1, so data on flow-frequency functions and stage-discharge functions must be obtained separately. 

Steps to Import Hydraulics

Import set of eight hydraulic profiles for the without-project condition, and for each with-project condition if the hydraulics are different than the without-project condition. The hydraulics data may be one of the three below imported from grids (.tif), or hierarchical data format (.hdf).

• Unsteady HDF
• Steady HDF
• Gridded Data

To import the hydraulics:

1. From the Study Tree, right-click on desired hydraulic dataset (e.g., Steady HDF) and select the import command.
    

2. For a given hydraulics data set, one HDF is required for steady hydraulics modeling, eight HDFs are required for unsteady hydraulics modeling. Gridded water surface elevation data is accommodated, see Hydraulics for more information. Specify a name, description, source file (or folder for unsteady), and be sure to identify either the correct Annual Exceedance Probabilities or correct Return Intervals.

3. For the Steady HDF example importer, entering the Return Interval values updates the annual exceedance probability value. 
4. Users also have the option to set the study projection. Select the Set Study Projection button to open the Study Properties dialog for importing the study projection file.